Several cuprate superconductors are capable of superconducting above the boiling point of liquid nitrogen (77° K). These cuprate superconductors (called high-temperature superconductors (HTSL)), however, have poor mechanical properties. The development of band lines is an attempt to overcome the associated problems.
Band lines (also know as band-HTSL or band-shaped HTSL) are coated conductors including a superconducting functional layer applied to a band-shaped substrate via a special process. The functional layer may include, e.g., yttrium-barium-copper-oxide YBa2Cu3Ox (YBCO). As shown in FIG. 1, these band lines have a structure including a metal substrate, a buffer layer, and a superconductor layer. The economic efficiency of the production process is decisively determined by the precipitation process. The main difficulty in the production of coated conductors is that the superconductor layer must have an extremely high degree of texture, that is, a high degree of crystallographic orientation. The individual crystallites of the layer should be tilted against one another only by a minimum value, since otherwise the superconducting properties are severely impaired.
To achieve such a high degree of texture, two different preparation processes may be utilized. It is common to both preparations that, before the superconducting layer is deposited, a textured buffer layer is produced and placed on the substrate. Thus, when the superconducting layer is deposited on the buffer layer, the texture (orientation) is transferred to the superconducting layer. In the two preparations, metal substrates are used, since this is the only way that the strength of the band lines necessary for later use in electrical technology can be achieved.
In the first preparation process, an untextured, crystallographically-non-oriented metal substrate formed from, e.g., Hastelloy® alloy is used. A textured buffer layer (i.e., a buffer layer with crystallographic orientation) is then applied to the untextured substrate. Such a direct deposition can be carried out only using physical coating processes under high vacuum (e.g., Ion Beam Assisted Deposition (IBAD) and Inclined Substrate Deposition (ISD)). Drawbacks of this process are high equipment costs (caused, for example, by the high vacuum pressure requirements) and a low deposition rate. In the second preparation, the metal substrate is already textured by special deformation and temperature treatment processes. The texture of the substrate can thus be transferred to the buffer layer and, in turn, to the superconducting layer deposited thereon. The advantage of this method is that no direct deposition processes must be used. Here, physical processes, such as Pulsed Laser Deposition (PLD) and Thermal Co-Evaporation (TCE) and chemical processes, such as Chemical Solution Deposition (CSD) and Metal-Organic Chemical Vapor Deposition (MOCVD) may be used. Again the PLD and TCE processes require high vacuum pressure (and thus high equipment costs), even though they provide higher deposition rates than direct deposition processes.
Chemical coating processes (e.g., Chemical Solution Deposition (CSD)) are economical relative to physical coating processes since they work at normal pressure (i.e., without the need for high vacuum pressure), while providing a higher deposition rate. FIG. 2 shows two CSD processes. As shown, on the laboratory scale, coating with CSD processes may be carried out as a “dip-coating” process (FIG. 2A), in which the substrate is immersed into a solution and pulled back out, or as a “spin coating” process (FIG. 2B), wherein several drops of the solution are applied to a substrate and distributed by rotating the substrate (centrifugal force spreads the solution on the substrate). For production of greater lengths, the substrate band can be drawn through a coating solution and then dried in a furnace. A diagram of such a system can be seen in FIG. 3. As shown, the system includes a rinsing (take-off) unit, a coating unit, a drying unit, and a winding unit. The subsequent reaction is carried out at a high temperature.
A Coated Conductor Architecture or Shift Sequence while eliminating the buffer layer is not possible from a physical standpoint, since this layer is required primarily as a diffusion barrier. On the one hand, the buffer layer is to prevent metal atoms from the metal band substrate (e.g., nickel) from diffusing into the superconductor layer during annealing, which would contaminate the layer and thus degrade its superconducting properties. On the other hand, the buffer layer acts as an oxygen barrier, thus also no oxygen can diffuse through the buffer layer to the metal substrate band in the subsequent annealing treatment. There, it would result in the formation of a metal oxide barrier layer, which can result in the flaking of the buffer and superconductor layers.
Because of the above-mentioned advantages, the use of a textured metal substrate band, on which a buffer layer and the actual superconducting layer are applied by chemical deposition, is preferred as a production process for coated conductors. In this case, the individual deposition steps are followed by an annealing treatment, in which the deposited materials are crystallized to form texture that is transferred to the subjacent layer or the substrate. This process is a so-called “all-solution” process, which originates, relative to the coatings, only from solutions of individual components and a coating at ambient pressure.
While yttrium-barium-copper oxide (YBCO) is typically used in forming the superconducting layer, many compounds can be used to form the buffer layer. The basic requirement is the property of being deposited in a textured fashion and of passing on this texture to the superconductor layer. In addition to single layers, multilayer buffer layer systems are also used. Typically used buffer layer materials include yttrium-stabilized zirconium oxide, gadolinium zirconate, yttrium oxide, lanthanum aluminate, lanthanum zirconate, strontium titanate, nickel oxide, cerium oxide, magnesium oxide, lanthanum manganate, and strontium ruthenate.
To date, no band line having a high elastic current density similar to band lines in which at least one layer was applied by means of physical methods (e.g., Pulsed Laser Deposition (PLD)) could be produced using the “all-solution” processes. Producing Coated Conductors via CSD processes has been unsuccessful in the making of buffer layers capable of transferring their texture to the superconductor layer. It has been shown that even a deposition of the superconductor layer with physical methods (which results in demonstrably high-quality layers on physically deposited buffer layers) results only in a slight texture of the superconductor layer on CSD buffer layers and, as such, results in poor superconducting properties. This can be substantiated by a lack of texture transfer.
In addition to providing poor texture transfer capability of CSD buffer layers, depositing buffer layers via CSD processes starts mainly from solutions on which 2-methoxyethanol is based as a solvent. This solvent is classified as toxic, embryotoxic, and fertility-damaging and, therefore, is rather unsuitable for laboratory use as well as for technical applications. In addition, the starting substances for the example of the production of La2Zr2O7 (lanthanum-isopropoxide and zirconium-n-propoxide) are moisture-sensitive, so that the production of the solution must take place under inert atmosphere.
Another drawback of CSD processes for applying the buffer layers are the required high temperatures for crystallization of the buffer layers, which often lie considerably above 1000° C. and thus greatly limit the selection of suitable metal substrates. In addition, even in the case of substrates that have a sufficiently high melting point or softening temperature, the diffusion speed at temperatures above 1000° C. are so high that high levels of contaminants of diffused-in metal atoms from the substrate can be detected in the buffer layers.